The present invention relates to methods for maintaining the fuel pellets within a nuclear fuel rod under compression and particularly relates to methods for installing a locking retainer in a nuclear fuel rod to maintain a predetermined axial preload on the fuel pellets within the fuel rod.
In U.S. Pat. No. 4,871,509 issued Oct. 3, 1989, of common assignee herewith, there is illustrated a fuel column retainer using a coiled spring to maintain the fuel pellets under compression within the nuclear fuel rod. Two designs are disclosed in that patent. Both disclosed designs require features on a coil spring so that torsion can be applied by an installation tool to the upper coils of the retainer spring. The torsion reduces the coil diameter of the upper coils allowing the spring to be located inside the fuel rod. On the upper end, both designs of that patent have a tang formed by the end of the spring coil extending axially from the upper coils. On the bottom end, one design has a wafer with a slot that is welded to the bottom of the spring. The other design has a partial coil in between the upper and lower coils. These features however render the retainer difficult and expensive to produce and complicate the installation process.
More particularly, the installation tooling in that patent includes a rod that extends through the upper coils with a feature on the bottom to engage either the slotted wafer or the partial coil. The rod extends through a sleeve that has a slot designed to engage the axial tang above the upper spring coils. With both the sleeve and the rod engaging the spring features, the sleeve can be rotated relative to the rod applying torsion to the upper coils and reducing their diameter to enable insertion in the fuel rod. This is a complex and time consuming process in that the spring must first be loaded onto the tool then the parts of the tool are rotated to allow insertion. The tool must also then be located in the proper axial position while the torsion between the rod and the sleeve is maintained. Torsion can then be released and the tool withdrawn. The process is cumbersome, time consuming and difficult to automate. It also typically slows down the assembly of the fuel rod. Accordingly, there is a need for a new and simplified design and installation process and which process includes a spring and tooling which are low cost and have the capacity for automation.